There is a clear trend in the use of a terminal device (e.g. a mobile communication device) from voice-only communications to increased data transfer. In some cases, nonlinear systems may be utilized to accommodate a resulting increased demand on network capacity. For example, nonlinear systems may implement modulation methods with non-constant amplitudes to account for increased demand on network capacity. However, when these nonlinear systems are utilized, current consumption may increase, resulting in power being drained at a faster rate from a power supply of the terminal device.
Power amplifiers in a terminal device consume a considerable part of the total system power. It is desirable that at least some of the power amplifiers, such as a power amplifier of a transmitter section, are simultaneously efficient and linear in operation. This goal is generally difficult to achieve as power amplifiers tend to have their highest efficiency at maximum output power (i.e. operating in a saturation region). However, the output signal of the power amplifiers generally becomes increasingly nonlinear as the amplifiers go increasingly further into saturation.
In some instances, a power amplifier may provide a linear output signal when the output power of the power amplifier is reduced from the saturation region. However, the efficiency of the power amplifier may be reduced in these instances. Efficiency of the power amplifier is often important in terminal devices due to a relatively limited battery supply. In addition, optimizing efficiency of base station power amplifiers may also be important because power consumption may be a significant factor in their operating cost. Another technique to achieve a higher power output that is also linear is to increase the size of the power amplifier, although this results in a larger die size and usually higher cost.
To reduce cost, transmitters for terminal devices may include multiband, multimode solutions having one common broadband transmit path for low band and one common broadband transmit path for high band. With current technology, these lower cost implementations may need to be designed with considerable performance margins in order to fulfill all specifications. Consequently, these implementations may consume more current in comparison to designs having several narrow band paths in parallel.
Power amplifiers for mobile communication terminal devices typically use analog design techniques to improve the trade-off between linearity and efficiency. Several strategies are used which may also be combined. Analog design techniques are based on adjusting circuit characteristics which are sensitive to changes in operating conditions (e.g., temperature, battery voltage, frequency range, etc.) and may cause failure of the circuit if operating conditions change too much. Some analog design techniques compensate the amplitude and phase nonlinearities within a stage of a power amplifier by adjusting the bias, input impedance and/or output impedance. Further improvements may be achieved by designing the remaining nonlinearities of the individual stages to be opposed to each other, such that they cancel each other in a multi stage power amplifier. In this way, maximum linear power output and efficiency may be increased.
However, there is a clear trend among terminal component manufacturers to strive for higher and higher functional integration on a single silicon die, and towards single chip radios which may include the power amplifier; but analog design techniques are of limited use for modern scaled nanometer (nm) CMOS and BiCMOS technologies. In particular, standard nanometer CMOS and BiCMOS technologies typically have low breakdown voltage characteristics and strong device nonlinearities. In addition, analog design techniques may also be limited with respect to the broad band, multiple bands, and multiple standards covering currently developed terminal solutions.
Compared to previous analog linearization techniques, digital predistortion shows very good adaptability to changing operating conditions. In some cases, digital predistortion is utilized in base station power amplifier systems and are optimized for very high linearity. However, the closed loop systems used in base station power amplifiers are very complex and costly. Thus, the closed loop digital predistortion techniques used for base station power amplifiers cannot be easily adapted to mobile terminal devices. Additionally, some attempts to use digital predistortion in terminal device transmitters utilizing open loop configurations, which are configurations where output signal feedback is not used, failed mainly due to the impact of complex operating conditions and sample variations on system characteristics and the high calibration effort required.